1. Field of the Invention
The present invention relates to a process for chemical vapor deposition (CVD) of a superconductive oxide, more specifically a process for CVD of a superconductive oxide of (a rare earth metal or a metal of the VA group of the periodic table)-(alkali earth metal)-(copper)-(oxide) system on a substrate.
2. Description of the Related Art
The speed of computers has been remarkably increased, and a multiplication of processors, increase of the switching speed of devices, and a high density packaging of such devices for shortening the length of wiring are carried out to cope with this increase of the speed of computers. A high density wiring or interconnection necessitates fine wiring or interconnection patterns, which decreases the sectional areas of conductors used for the wiring or interconnection but increases the electrical resistance of the wiring or interconnection. This lowers the speed of an electrical signal transmission and distorts the wave-shape thereof.
If a superconductive material can be used as a material for wiring instead of a normal conductor such as copper, the above-mentioned problems will be quickly solved, and if a Josephson element and the like are formed with a superconductive material and integrated, the high speed and low electric power consumption thereof, in combination with a fine packaging art, will allow the realization of a super high speed computer system.
Conventional superconductive materials need a low temperature for transition to a superconductive state and, therefore, must be cooled by liquid helium or liquid hydrogen. Since these cooling mediums are difficult to handle and are expensive, it is practically difficult to use these superconductive materials.
Nevertheless, high temperature superconductive materials, represented by Y-Ba-Cu-O system oxide or ceramics, have been recently developed, and this has opened up new possibilities in the utilization of superconductive materials.
Since oxide superconductors exhibit a superconducting behavior at a relatively high temperature, i.e., higher than the boiling point of liquid nitrogen (77K), oxide superconductors can be widely utilized in, for example, semiconductor devices such as IC's, as parts of various devices, and as wiring in devices a strong demand has arisen for such superconductors. To satisfy this demand, it is necessary to efficiently form a high quality thin film. For example, a semiconductor integrated circuit is composed completely of thin film elements, including a Josephson element, and as a result, the characteristics of a thin film, which depend on the crystallinity thereof, such as crystal size and crystal orientation of the thin film, and the uniformity and reproducibility of the thin film, are important factors determining the yield and reliability of elements and an integrated circuit.
Conventional methods of forming a thin film of a semiconductor material include sputtering and evaporation. In the sputtering process, a target having a composition similar to that of a material to be deposited is used and is vaporized by ion sputtering to be deposited on a substrate. In the evaporation process, a material (source) for forming a thin film is heated until evaporation occurs and is deposited on a substrate.
These conventional thin film forming methods may be applied to a high temperature superconductive material but it is difficult to provide a good crystallinity thin film, particularly a single crystalline thin film, thereby. The sputtering method is suitable for forming a thin film of a single element (Si or a metal) or a simple compound which is not easily decomposed (SiO.sub.2, Al.sub.2 O.sub.3, etc.), but is difficult to form a thin film of a complex compound by sputtering, since such a compound is decomposed by the sputtering, and thus control of a composition is very difficult. In the evaporation process, if a compound composed of multi-elements is used, it is difficult to form a film having a uniform composition, since an element which is easily evaporated is first evaporated and a material which is not easily evaporated remains. Particularly, it is difficult to evaporate a compound such as an oxide and deposit it in a uniform manner. In this regard, a method has been proposed of depositing a film of metals having a ratio between the metals required for a desired metal oxide, followed by oxidizing the metal film. In this method, however, the volume of the film is changed by oxidation, which causes roughness of the surface of the film, peeling of the film, or a nonuniform film quality, and it is difficult to obtain a dense film thereby.
In either the sputtering or the evaporation process, it is difficult to form a single crystalline film having a complex composition, since a deposition of discrete metals, alloy and metal oxides or compounds on a substrate occurs. For example, as seen in FIG. 1, when a superconductive oxide of the Y-Ba-Cu-O system mentioned above is formed, discrete metals such as Y, Ba and Cu, various discrete oxides or compounds such as Y.sub.2 O.sub.3, BaO.sub.2, Y-Ba-O, CuO, Cu.sub.2 O, Y-Cu-O, Y-O, Ba-Cu-O and Ba-O, or alloys, are deposited on a substrate, and as a result, it is difficult to obtain a film of a compound having a desired composition or a good crystallinity, and such a film has a disadvantageously decreased current density and reduced boundary characteristics when made into a fine pattern, and the like, and thus it is not practical for application to a semiconductor integrated circuit.